Plasma-enhanced chemical vapor deposition (PECVD) is an established technique for thin film synthesis due to its ability to deliver high rates at low thermal budget. PECVD offers a suite of parameters, including pulsed power modulation (Overzet, L. J. and Verdeyen, J. T., 1986), that can be tailored to meet specific demands. Pulsed PECVD has been adopted for silicon (Biebericher, A. C. W., et al. 2000) and carbon-based (Mackie, N. M., et al. 1997; Bauer, M., et al. 2005) materials, with demonstrated improvements in composition (Labelle, C. B., and Gleason, K. K. 1999), rate (Overzet, L. J. and Verdeyen, J. T.; Biebericher, J., et al. 2000), and performance (Mukherjee, C., et al., 1995; Fujioka, Y., et al., 2006). In the case of oxides most pulsed PECVD work has been directed at SiO2, where its attributes include improved uniformity, reduced film stress, and enhanced dielectric performance (Charles, C. 1998; Goto, H. H., 2002).
Metal oxide thin films serve as high κ replacements for SiO2 (Maeng, W. J. and Kim, H. 2006; Cho, M., et al. 2004), and as diffusion barriers (Carcia, P. F., et al. 2006). Atomic layer deposition (ALD) is an attractive technique due to its potential to deliver high quality films with monolayer thickness control. In metal oxide ALD, a precursor and an oxidizer are sequentially exposed to a heated substrate, separated by inert gas purges. Although ALD has been demonstrated for nearly every metal oxide (George, S. M., et al. 1996; Puurunen, R. L., 2005), the surface chemistry of the metal precursor can often be the rate-limiting step in both the thermal and plasma-assisted modes. Species with low reactivity require large exposures to achieve surface saturation and steric effects can limit surface coverage for large precursors. Metal ligands are a leading source of impurities that impact performance. Finally, monolayer deposition is usually limited to a small (˜100° C.) temperature range. For example, in Ta2O5 ALD using penta-ethoxy tantalum (PET, Ta(OC2H5)5) and H2O, self-limiting growth is achieved between 225-300° C., rates are ˜0.4 Å/cycle, and the resulting films are poor dielectrics with leakage current densities >1 mA/cm2 at 1 MV/cm electric field (Kukli, K., et al, 1995; Lee, Y. H. et al., 2004).
Atomic layer deposition (ALD) was discovered almost 30 years ago (Suntola, T. and Anston, J., 1977), but only in the past decade has it risen to prominence as increasing performance requirements have demanded the control and quality at the nanoscale that only ALD can provide. Since its inception, ALD researchers have focused on the chemical aspects of this process. They have identified numerous combinations of reactants (George, S. M., et al. 1996), developed new classes of metal precursors (Lim, B. S., et al., 2003; Maeng, W. J. and Kim, H., 2006), and explored numerous oxidizing agents (Ritala, M., et al., 2003; Cho, M., et al., 2005). In general, the basic process has remained unchanged, employing the A/purge/B/purge sequence where 50-90% of the total cycle time is spent on purge steps.
Atomic layer deposition is a process that provides monolayer control to chemical vapor deposition (CVD) processes (George, S. M., et al. 1996; Goodman, C. H. L. and Pessa, M. V., 1986). ALD has been used to deposit metals (Jezewski, C., et al., 2005), oxides (Lai, S. X., et al., 2005, nitrides (Park, J-S, and Kang, S.-W., 2004), and carbides (Kim, D. H., et al. 2003). The process involves alternating exposure to two different reactants as shown in FIG. 1. In the case of metal oxides, precursor A is typically an organometallic precursor that saturates the surface with a monolayer of coverage. Reactant B is an oxidizer (H2O, O3, O) that removes the organic ligands and converts the adsorbed metal into a film. Both reactions are self-limiting, and the process is repeated with films grown one monolayer at a time.
This process provides exceptional quality and precise control over film thickness. The self-limiting nature of the reactions ensures large area uniformity, and conformal coverage on complex structures has been achieved. It is critical that the two reactants do not meet, and in order to prevent gas-phase chemistry the two are separated using inert purge cycles as shown in FIG. 1b. The process requires complex mechanical actuation of flow rates, and expends significant quantities of carrier gas. The purge cycles can be rather long, particularly when using low volatility precursors and/or employing water as the oxidizer. A typical commercial process may require ˜5 s/cycle. Combined with deposition rates of ˜1 Å/cycle the overall deposition rates remains quite low (˜1 nm/min). The present proposal advocates a simpler process that offers much higher rates through reduced cycle times while maintaining ALD quality and control.
The fast growing ALD market is on its way to becoming a billion dollar/year industry (Electronics.ca Research Network, 2003). One large growth area for ALD is the manufacturing of organic light emitting diode (OLED) displays. These displays are currently finding growing market share as replacements for liquid crystal technology in mobile phones and mp 3 players (reed-electronics.com, 2005). In 2005, 61 million OLED display units were produced, a market growth of 65 percent. By 2011, the OLED market is expected to reach 341 million units valued at $2.9 billion. The use of metal oxide barriers such as Al2O3 deposited by ALD dramatically reduces up water vapor penetration that is detrimental to OLED lifetime (Park, S. H. K., et al., 2005). Additionally, the ALD growth of high κ dielectrics and semi-conducting oxides could improve the performance of thin film transistors (TFT) that switch these devices (Carcia, P. F., et al., 2006).
ALD is used extensively to deposit thin films for both microelectronic and optoelectronic applications. The need for ALD will continue to grow as device dimensions and tolerances continue to shrink. A $1 billion dollar industry in 2005, it is expected to grow to $2.9 B by 2011. ALD provides unparalleled quality and thickness control, however its low deposition rates (˜1 nm/min) preclude ALD from competing with conventional thermal and plasma-enhanced chemical vapor deposition (CVD/PECVD) techniques in a number of markets. Leading commercial ALD systems require cycle times of ˜5 s. A method is needed to decrease this value by an order of magnitude, and reap the commensurate increase in throughput.